Parents or grandparents can buy insurance on behalf of dependents. Dependent children- mean- Your financially dependent children or grandchildren (including fostered, or adopted children or grandchildren) who are under 18 years of age and who are named on the Policy Schedule. That basically means, 17 or under can travel for free, as long as travelling with parents. However, if the child is 18 or above, he/she will be charged as adults, whether or not a couple/family policy.
Parents or grandparents can buy insurance on behalf of dependents as long as dependents under 16 years of age will have full 100% adult supervision during the trip.